/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     struct TreeNode *left;
 *     struct TreeNode *right;
 * };
 */
bool isSameTree(struct TreeNode* p, struct TreeNode* q) {
    if (p == NULL && q == NULL) return true;
    if ((p == NULL && q != NULL || p != NULL && q == NULL)
        || (p->val != q->val))
        return false;
    return isSameTree(p->left, q->left) && isSameTree(p->right, q->right);
}